The efficacy of ferrocene, a volatile red solid, as a burning rate accelerator in a solid composite propellant, was discovered in the early 1950's. It was found that ferrocene was superior to the metallic compounds, such as iron oxide, copper chromite and the like, then in use. Ferrocene, in equivalent amounts, gave much larger increases in burning rate and could be used effectively in increasingly higher concentrations with concomitant increase in burning rate.
Unfortunately propellants containing ferrocene undergo changes in composition with time due to volatility of the catalyst compound. This results in changes in both mechanical and ballistic properties during storage. Rocket motors containing ferrocene--catalyzed propellant grains were observed to have red needles of ferrocene sublimed and recrystallized on the grain surface.
Efforts were then turned to development of liquid ferrocene derivative catalysts having higher molecular weight and decreased volatility as compared with ferrocene. In addition to reducing the ferrocene volatility problem, the liquids improve processing properties by reducing the total amount of added solids and functioning as a plasticizer. However, two serious difficulties were encountered with the liquid ferrocene derivatives, crystallization and migration.
Crystallization of the liquid at low temperatures increases the solids content of the propellant above the design concentration and can, thereby, adversely affect mechanical properties.
The liquid also tends to diffuse from the propellant into the rubbery materials normally used in making the conventional liners employed with solid rocket propellant grains. This results in embrittlement of the propellant and undesirable modification of ballistic properties adjacent to the interface between the propellant and liner.
In view of these problems with ferrocene and liquid derivatives thereof, efforts have been made to find a solid ferrocene derivative replacement for the highly volatile ferrocene. To be successful, such a compound must meet several essential criteria. It must produce a substantial increase in burning rate of the propellant as compared with the non-catalyzed composition. It must be a stable, substantially non-volatile compound.
It must not adversely affect the physical or ballistic properties of the propellant composition in such terms, for example, as weight loss due to volatilization or decomposition at the environmental temperatures to which the propellant grain will be exposed, including substantially elevated temperatures; migration or diffusion; increase in propellant sensitivity to friction, impact or heat; production of ballistic unpredictability, such as variation in burning rate within the propellant grain; and the like.
A number of solid, relatively stable ferrocene derivatives have been tried as propellant burning rate accelerators. These include, for example, dimethyl polyferrocenyl methylene (DMPFM), a polymer produced by the reaction of ferrocene and acetone; 1,3-diferrocenyl-1-oxo-2 propene; 1,3-diferrocenyl-1,3-propanedione; and benzoyl ferrocene. DMPFM appeared to be one of the more promising solid ferrocene derivatives for use as a catalyst because of its stability per se and minimal adverse effects on propellant stability. However, it has been found to be inadequately effective as a burning rate accelerator. Other solid substantially stable ferrocene derivatives, which have been tried as propellant burning rate catalysts, produce grains having unacceptable physical and/or ballistic properties.
It has been suggested in the ferrocene derivative chemical art that a carbonyl group attached to the ferrocene moiety increases the thermal and oxidative stability of the compound. This, however, does not suggest the effect of a particular derivative when included in a propellant formulation.
Diferrocenyl ketone also known as 1,1" -carbonyl-bis-ferrocene is a known chemical compound. It has not, however, been used or suggested for use as a propellant burning rate catalyst. Given the inadequacy of other solid ferrocenyl compounds as burning rate catalysts, it is surprising to find that diferrocenyl ketone is not only stable as a compound, but also substantially increases burning rate without adverse effects on the stability of the propellant.